In chronic renal disease, the portions of the kidney that are relatively undamaged are responsible for maintaining kidney function; yet deterioration often ensues, even in the absence of activity of the primary disease. Similarly, in experimental renal insufficiency induced by subtotal nephrectomy in the rat, there is at first good compensation; later renal tissue deteriorates with the appearance of hypertension, azotemia, and abnormal glomerular permeability with increasing proteinuria. It is the objective of this study to obtain ultrastructural data, expecially quantitative data, concerning the changes in the glomerulus and glomerular filter accompanying the progression of renal failure in this system. The effect of chronic peritoneal dialysis in modifying these morphologic changes will be studied, as well as the progress of the morphologic disease in another model of chronic renal failure, in which a functionally subtotal nephrectomy is surgically induced. Ultrastructural studies will be complemented by probe molecule studies with native ferritin and cationized ferritins. Morphometric studies of the glomerular mean caliper diameter, mean glomerular volume, the length density of the glomerular slit and surface density of peripheral basement membrane and endothelium will be compared in all three systems.